It is generally known that during the combustion of tobacco in smoking articles, many harmful substances are generated. Therefore there exists an interest in the industry to produce smoking articles the smoke from which contains considerably fewer harmful substances.
A smoking article, typically a cigarette, comprises at least one tobacco column, which is wrapped with a wrapping material. In many cases smoking articles are also equipped with filters to influence the type and amount of substances in the smoke. Such filters, mostly made out of cellulose acetate or paper, can reduce the particulate fraction of the smoke. Filters can also contain other substances, such as activated carbon or flavorings.
The amount and type of substances generated during smoking of smoking articles are determined by a method whereby the smoking article is smoked under standardized conditions. Such a method is described in ISO 4387, for example. In it, the smoking article is at first lit at the beginning of the first puff and then each minute a puff is taken at the mouth end of the smoking article with a duration of 2 seconds and a volume of 35 cm3 using a sinusoidal puff profile. The puffs are repeated until the length of the smoking article falls below a length that has been pre-defined in the standard. The smoke exiting the mouth end of the smoking article during the puffs is collected in a Cambridge Filter Pad. Afterwards, the Cambridge Filter Pad is analyzed chemically with respect to its content of various substances, for example nicotine. The gas phase exiting the mouth end of the smoking article through the filter pad during the puffs is collected and also chemically analyzed, for example to determine the carbon monoxide content.
During standardized smoking, the smoking article is in two different states of flow. During a puff, a considerable pressure difference is generated, typically in the range 200 Pa to 1000 Pa between the inner face of the wrapping material, facing the tobacco, and the outer face. Due to the pressure difference, air flows through the wrapping material into the tobacco part of the smoking article and dilutes the smoke generated during a puff. During this phase, which lasts for 2 seconds per puff, the extent of dilution of the smoke is determined primarily by the air permeability of the wrapping material.
In the period between the puffs, however, the cigarette smolders without any considerable pressure difference between the inside of the tobacco column and the surroundings, so that the gas transport between the tobacco column and the surroundings is determined by the gas concentration difference. As a result, carbon monoxide can diffuse through the wrapping material into the ambient air and oxygen can diffuse from the ambient air through the wrapping material into the tobacco column. In this phase, which last 58 seconds per puff according to the method described in ISO 4387, the diffusion capacity of the wrapping material is the relevant parameter for the gas transport.
Apart from the carbon monoxide content in the smoke of a smoking article, the diffusion capacity is also of great importance for the self-extinguishing smoking articles known in the prior art. In such smoking articles, burn-retardant stripes are applied to the wrapping material in order to obtain self-extinguishing in a standardized test (ISO 12863). This or a similar test is, for example, part of the legal regulations in the USA, Canada, Australia and the European Union. The self-extinguishing property is caused by the fact that the wrapping material has a substantially lower diffusion capacity in the region of the stripes than outside these stripes. As a result, the diffusion of oxygen from the surroundings through the wrapping material to the glowing cone of the cigarette is reduced, so that self-extinguishing occurs under certain conditions. The diffusion capacity of a wrapping material of a smoking article can be reduced either by imprinting stripes in the circumferential direction, for example made from starch, alginate, guar or similar materials known in the prior art. Alternatively, a wrapping material can be produced that, due to its composition, already exhibits an intrinsically low diffusion capacity. The areas of reduced diffusion capacity do not need to be present as stripes, instead they can have any other geometry compatible with any legally required self-extinguishing property.
The diffusion capacity is a characteristic property of a wrapping material of a smoking article. It describes the permeability of the material for a gas flow that is caused by a concentration difference. Therefore, it indicates the gas volume passing through the wrapping material per unit time, per unit area and per concentration difference, and hence it has the unit cm3/(cm2 s)=cm/s. A measurement of the diffusion capacity for carbon dioxide (CO2) can, for example, be carried out by means of a diffusion capacity measurement instrument from the company Borgwaldt KC (Diffusivity Tester) or Sodium (CO2 Diffusivity Meter).
A measurement of the diffusion capacity can be carried out according to the Recommended Method No. 77 issued by the Cooperation Centre for Scientific Research Relative to Tobacco (CORESTA). In this regard, the sample of the wrapping material is fixed in a measurement chamber after appropriate sample preparation and conditioning according to ISO 187, wherein the sample divides the measurement chamber it two halves of nominally identical geometry, which are only separated by the wrapping material. Carbon dioxide is passed into the first of the two chamber halves, while nitrogen is passed into the second half-chamber. Both gases should flow through the chamber with the same velocity parallel to the surface of the wrapping material and technical measures are taken to ensure that no significant pressure difference exists between both faces of the wrapping material. Due to the concentration difference, carbon dioxide diffuses from the first half of the measurement chamber through the wrapping material into the second half-chamber, while at the same time, nitrogen diffuses from the second half of the measurement chamber through the wrapping material into the first half of the measurement chamber. At the outlet from the second half-chamber, the volumetric concentration of carbon dioxide in the nitrogen flow is measured after reaching a stationary value. The diffusion capacity can be calculated from the volumetric concentration of carbon dioxide.
For smoking articles and particularly for smoking articles that should self-extinguish, there is often the desire to find a wrapping material where carbon monoxide can easily diffuse from the inside of the smoking article through the wrapping material into the surroundings, so that the carbon monoxide content in the smoke flowing out of the mouth end is low. This indicates a wrapping material with a comparably high diffusion capacity for gases. Conversely, it is also often desirable that oxygen should only diffuse with difficulty from the surrounding air through the wrapping material into the smoking article to ensure self-extinguishing of the smoking article in accordance with legal requirements. Thus, there exists a certain conflict of goals for the selection or design of a wrapping material with respect to its diffusion capacity.
In some cases the converse can be intended, i.e. that harmful gases such as carbon monoxide should remain in the tobacco column of the smoking article and only diffuse with difficulty through the wrapping material into the surroundings, to mitigate the harmful effect of passive smoking, while oxygen from the surroundings should be able to diffuse comparably easily through the wrapping material, to ensure continued smoldering of the smoking article and to reduce the rate of generation of carbon monoxide by increasing the availability of oxygen. In this case too, the result can be a conflict of goals, as described above, —but in reverse.